During the combustion of sulphur-containing fuels, such as coal or fuel oil, in power plants, sulphur dioxide-containing gases, i.e. so-called flue gases, are formed. The SO.sub.2 concentration in these flue gases is relatively low; for instance, if coal with a sulphur content of 1% is used, the SO.sub.2 concentration is about 700 ppm. Due to the large installed power plant capacity however, the absolute SO.sub.2 emission value is very high. For example, in a 600-MW-power plant on the basis of coal having a sulphur content of 1%; about 2.times.10.sup.6 flue gas is emitted per hour; this corresponds to an SO.sub.2 amount of approximately 4 tons per hour (4t/h).
As a result of stricter environmental regulations, a variety of methods for the desulphurization of flue gases have been developed in the past few years and have also been employed in industrial countries. In the majority of the processes, the sulphur dioxide is ultimately converted into calcium sulphate (gypsum), part of which is disposed of as waste products, part of which is used in the building material industry. The use of said calcium sulphate in the building material industry, however, is limited on account of the demand in this field. The disposal of calcium sulphate is linked to the problem of environmental pollution. The hardness of the ground water in the vicinity of the disposal site increases significantly. What is more, as power plants are under compulsion to desulphurize flue gases, the amount of calcium sulphate will increase considerably during the coming years.
Furthermore, methods are known for the reduction of sulphur dioxide-containing gases to sulphur using coke (gas generator method) or of gases containing hydrogen, carbon monoxide, methane and other hydrocarbons; cf. Ullmanns Enzyklopadie der technischen Chemie, 4th edition, Vol. 21, 1982, page 24. One particular method of this kind is the SO.sub.2 reduction in a modified Clausplant. This method is mainly used for the flue gas purification of SO.sub.2 quantities above about 5t/h. The method has the disadvantage that large investments have to be made in the plants, that the handling of the method for the reduction of SO.sub.2 is complicated and that it can hardly ever be used for small quantities of flue gas.